Probably the greatest empire was the Roman Empire. It flourished for an unusually long time for an empire - about 400 years. It reached its height from 27 BC to AD 180. BC stands for before Christ. AD stands for anno Domino, which is Latin for in the year of our Lord.

The last great empire was the British Empire of the 19th century. It stretched so far around the globe that it was said that the sun never set on the British Empire. At one time it embraced a quarter of the world's population.

But like all empires it faded out and folded soon after World War II, which ended in 1945.

That war started on Sept. 1, 1939, when Nazi Germany invaded Poland. Two days later, Britain and France went to war against Germany.

When the war ended, Germany was defeated and lost its empire. But the war also took such a toll on France and Britain that after the war they too found it necessary to give up their empires.

It was the United States and the Soviet Union that emerged as the great new powers after World War II.

In a sense, the end of World War II saw the rise of what we might call the Soviet Empire. Some people call it the world's last real empire.

The Soviet Union was so huge in its area, its population, its resources, and armed power that it dwarfed the rest of Eastern Europe.

It wasn't long before the entire region came completely under Soviet control.

This is what Sir Winston Churchill, the great British wartime prime minister, meant when he said in a speech in Fulton, Mo., in 1946 that ''an iron curtain has descended across the Continent.'' He was referring to Europe. As a result, he made popular the phrase ''behind the Iron Curtain.'' It refers to all those communist countries in Eastern Europe shut off from the democracies of the West because they had come under the armed might and political control of the Soviet Union.

The Soviet Union expanded its empire after World War II by taking in parts of Finland, Poland, and Romania. A few years later three entire countries were taken over and forced to become an essential part of Soviet territory. These were Latvia, Estonia, and Lithuania. For a short time before the war, they were independent nations. Now they form three of the 15 republics that make up the Union of Soviet Socialist Republics (USSR). Here Socialist means Communist. Russia is another name for the Soviet Union or USSR but it is not really a correct one since Russia forms only a part of the larger area known as the Soviet Union.

More important in some ways than the adding of chunks of new land to the Soviet empire was the way in which the Soviet Union stretched its political control over all East Europe.

This was true of Poland, Czechoslovakia, East Germany, Hungary, and Bulgaria. Although they are independent countries, they have come almost completely under the thumb of the Soviet Union. They act at Soviet bidding. That is why we call countries of Communist East Europe or the East bloc ''satellites'' of Moscow. Moscow is the capital of the Soviet Union.

The Soviet Union expects them to remain communist and to protect the communist system in East Europe.

To do this the Soviet Union and these East European countries formed a military alliance or grouping in 1955. This communist alliance is called the Warsaw Pact after Warsaw, the capital of Poland.

Noncommunist Western Europe, together with the US and Canada, has its own military alliance. It's called NATO, which is short for North Atlantic Treaty Organization. NATO and the Warsaw Pact are not at war. But they recognize each other as the enemy.

As big and powerful as the Soviet Union is, it has not always been able to keep all its allies in line. Yugoslavia broke away from Soviet domination in 1948. It is not a member of the Warsaw Pact even though it remains communist. Another East European country that has gone its own way is tiny Albania. It, too , is communist, but it is not a member of the Warsaw Pact and does not want to be controlled by Moscow.

Even in the rest of Eastern Europe the Soviet Union has been forced from time to time to put down rebellions and unrest.

Right now Poland is in turmoil over the communist system. Some people think we may be starting to see the eventual breakup of the Soviet empire.